Change-pole motors are asynchronous motors, whose stator houses more than one winding system. By switching in the particular stator winding, they make it possible to operate at different output speeds. The speeds result from the rotary frequency and the number of poles and are therefore in an integral relationship to one another. In general, the stator windings are independent, physically separated windings.
The different uses require not only the switching in and out of each individual speed, but also the switching between the speeds. The switching in of each speed with the motor stationary only differs from a single-revolution motor in that the run-up time is greater as a result of the relatively higher dead weight of the change-pole motor. The switching between the speeds and in particular the switching back to the lower speed represents a new situation with a behavior typical to a change-speed motor. There is a switching back torque, which reaches a multiple of the starting torque. This moment surge effects the drive mechanism through increased wear and vibrations and by increased noise evolution. For physical reasons the speed moment pattern is much higher in supersynchronous than in subsynchronous operation. This more particularly applies for the generating pull-out torque, which significantly determines the hardness of switching back.
In order to avoid or reduce to an acceptable amount the above-described disadvantages when switching back change-pole motors, a number of different technical measures are used.
In two-phase operation a running rotary field motor develops a torque, which is smaller than in three-phase operation and is elliptical. The choice of the two phases has no influence on the torque pattern. The two-phase switching back method achieves the desired moment reduction in the supersynchronous operating range. The problem is that immediately prior to synchronous speed it is necessary to switch back to three-phase operation.
If the switching in of the third phase takes place too early and therefore during the generating pull-out torque, then the switching back surge is only inadequately reduced. However, if switching in takes place too late, this means that the drive operates with a greatly reduced torque during motor operation and this is well below the nominal torque. This state must be reliably avoided particularly in the case of lifting gear.
The effectiveness of the described method is significantly determined by the accuracy of the switching in time of the third phase. The least effort and expenditure occur with the time-delayed switching in, but this only functions in a satisfactory manner in exceptional cases, because the duration of the braking phase is rarely constant in practical operation. The switching in as a function of the measured speed takes place much more accurately and functions freely from external influences. However, the disadvantage of this method is the technical expenditure with respect to the measuring device and the installation.
This method with a "heavy fan" or an additional flywheel mass is often used, because the technical expenditure is low and it requires no additional control. Thus, there are no corresponding contactors, electronic controls, as well as adjustment and installation. The additional flywheel mass is frequently in the form of the actual fan.
Operation is based on the fact that the additional flywheel mass is a dynamic energy store and reduces the acceleration processes during speed switching. It is disadvantageous that this process not only acts during switching back, but also during each starting process. A further and very significant disadvantage is based on the increased losses. During each starting and switching process the motor function in tilted operation and, consequently, suffers from losses caused by the very principle and these are exacerbated during longer starting and delay phases. It is not possible to recover the kinetic energy from the flywheel mass.
The permitted operating frequency of motors used in timed operation decreases considerably or requires additional measures with respect to the thermal stability and additional cooling. Apart from the technical costs involved, the running costs with respect to the increased energy losses constitute an important disadvantage of this method.